A particulate solid activator comprising a porous material containing absorbed P2 O5 values can be prepared by a process which comprises spray drying a slurry of a liquid P2 O5 material and the porous material to obtain the particulate solid. The P2 O5 material can be a mixture of a phosphoric acid and an ammonium phosphate solution. The porous material can be diatomaceous earth. The resulting particulate solid activator can be dry blended with a magnesium-containing compound, e.g. MgO, and an optional aggregate to form a magnesium phosphate fast-setting cement mixture. This mixture reacts in the presence of an aqueous component at ambient temperature to form a monolithic solid.
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1. A process for preparing a particulate solid activator capable of reacting at ambient temperatures with a magnesium-containing component in the presence of an aqueous component to form a magnesium phosphate fast-setting cement comprising diatomaceous earth containing absorbed P2 O5 values, said process comprises spray drying a slurry of an ammonium phosphate solution, phosphoric acid and the diatomaceous earth to obtain said particulate solid activator.
2. The process of
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1. Field of the Invention
This invention relates to fast-setting magnesium phosphate cements. More particularly, it relates to an improved process for the preparation of a particulate solid activator comprising a porous material containing absorbed P2 O5 values which, when admixed with MgO, reacts in the presence of an aqueous component at ambient temperatures to form a magnesium phosphate fast-setting cement.
2. Related Developments
Cementitious compositions having fast-setting time and prepared by mixing a magnesium-containing component with a P2 O5 material and an optional aggregate are popular for use in repairing or patching cracks, holes and spalled areas of highways, airfield pavements and bridge decks, as well as for a variety of commercial and industrial applications.
U.S. Pat. No. 3,202,520 (Enoch, Aug. 8, 1965) discloses a rapid set non-hygroscopic cement composition consisting essentially of a blend of a phosphate derived from phosphoric acid and of alumina, and containing from 15 to 55 weight percent of P2 O5 and from 45 to 75 weight percent of alumina, and from 5 to 18 weight percent of magnesium oxide. The phosphoric acid and alumina are blended at room temperature, with the alumina added to the liquid phosphoric acid. The resultant mixture is dried above 100°C for a sufficient time to remove all moisture. The resulting dried material is milled to pass through a 325 mesh screen prior to being dry blended with the MgO.
U.S. Pat. No. 3,525,632 (Enoch, Aug. 25, 1970) discloses a rapid setting concrete cement composition which comprises dry blending magnesium-containing compounds, aluminum-containing compounds and phosphorus-containing compounds from stated sources, in stated weight percentages, and then pulverizing the blended material without a separate drying stage. The phosphorus-containing compound is a mixture derived from phosphoric acid, and at least 50 weight percent of one of trimagnesium phosphate, aluminum orthophosphate, and phosphoric anhydride. This reference states that the compositions of U.S. Pat. No. 3,202,520 (discussed above), which require a drying step to remove moisture, resulted in an increased cost. This drying step is carried out in expensive stainless steel equipment to avoid corrosion and contamination problems. Further, during drying, there is a pronounced tendency for the mixture to cake within the dryer causing serious production problems.
U.S. Pat. No. 3,879,209 (Limes et al., Apr. 22, 1975) discloses a process for a fast-setting concrete comprising establishing a mixture of an aggregate, containing at least 10 percent by weight of magnesia, and ammonium phosphates in aqueous solution. This patent discloses that a commercial agricultural fertilizer product known as sequestered phosphatic solution (SPS) is particularly suitable as a source of the ammonium phosphate. No separate drying of the SPS is disclosed in this reference.
U.S. Pat. No. 4,059,455 (Limes et al., Nov. 22, 1977) discloses a process for making a fast-setting concrete comprising establishing a mixture of an aggregate, containing at least 1 percent magnesia, and ammonium phosphates in aqueous solution. No separate drying of the ammonium phosphate solution is disclosed in this reference.
U.S. Pat. No. 4,174,227 (Tomic, Nov. 13, 1979) discloses a grouting system comprising an acidic reactive component comprising at least one acidic oxyphosphorus compound selected from phosphoric acids, anhydrides of phosphoric acids and salts of phosphoric acids with multivalent metal cations, and a basic reactive component comprising at least one basic metal compound of a Group II or Group III metal capable of reacting with the oxyphosphorus compound(s) in the presence of water to form a monolithic solid. This reference does not disclose a separate drying step for the acidic reactive component.
U.S. Pat. No. 3,821,006 (Schwartz, June 28, 1974) discloses a patching composition consisting essentially of a reactive component of MgO with an acid phosphate salt and an inert aggregate component. The particle size of the inert aggregate component is related to the compressive strength of the cement formed at an early cure age. This reference does not disclose a separate drying step for the acidic component.
The mixtures employed in the methods of the references discussed above, set and develop usable strength much more rapidly than conventional materials. Nonetheless, each of these references is limited, as discussed above, and therefore distinguishable from the process of the present invention in several ways.
U.S. Pat. No. 3,673,111 (Hovarth, June 27, 1972) discloses a process for manufacturing a solid phosphoric acid catalyst in which a siliceous adsorbent is admixed with a polyphosphoric acid. The mixture is heated at an elevated temperature and thereafter extruded. The extrudate is further treated by drying first in a steam atmosphere and then in a dry air atmosphere. This reference does not teach a process for producing solid P2 O5 material suitable for use in fast-setting cements but rather teaches a process for making a catalyst.
U.S. Pat. No. 3,475,188 (Woodhouse et al., Oct. 28, 1969) discloses a dry refractory composition consisting essentially of phosphates absorbed on diatomaceous earth, up to 80 percent by weight being absorbed thereon, magnesium oxide and refractory aggregate. Compositions and processes in which well above 80 percent by weight of phosphates absorbed on diatomaceous earth are present would be advantageous. A separate drying step for the phosphates absorbed on diatomaceous earth is not disclosed. This reference is directed to the refractory arts wherein curing at high temperatures of the phosphate/MgO blend is contemplated before substantial compressive strength develops. This invention is directed to the fast-setting magnesium phosphate cement art wherein the phosphate/MgO blend reacts at ambient temperatures in the presence of an aqueous component to yield substantial compressive strengths shortly after hydration.
U.S. patent application Ser. No. 617,317, filed June 7, 1984, which is a continuation-in-part of U.S. patent application Ser. No. 522,077, filed Aug. 11, 1983 and now abandoned, discloses a particularly effective solid phosphorus pentoxide material suitable for use in fast-setting magnesium phosphate cements. This material is prepared by a process which comprises mixing a porous material with a liquid phosphorus pentoxide material and heating the mixture until a dry solid is produced. This solid phosphorus pentoxide material can then be dry blended with a solid component comprising magnesium oxide, magnesium hydroxide, magnesium carbonate or mixtures thereof. An aggregate can optionally be added to this dry blend. When hydrated, this dry blend has been found to be particularly advantageous as a fast-setting magnesium phosphate cement. An aqueous solution of ammonium phosphate has been found to be particularly advantageous when used as the phosphorus pentoxide material that is mixed with the porous material. Heating this mixture of ammonium phosphate and the porous material to form a dry solid can yield inconsistent results, e.g., the drying time can be extremely long and/or the resulting dry solid can be difficult or even incapable of being milled to a preferred mesh size.
U.S. patent application Ser. No. 735,907, filed by Tieckelmann et al., May 20, 1985, discloses an improved process for drying ammonium phosphate solution absorbed onto porous material which comprises adding an effective amount of an acid, preferably orthophosphoric acid, to the solution prior to the drying step. The improvement comprises a substantial shortening of the drying time. The resulting dry activator can be used in fast-setting magnesium phosphate cements.
A process for improving this drying process while maintaining the advantages of these particular compositions of magnesium phosphate fast-setting cements would be advantageous.
An object of this invention is to provide a process for preparing improved magnesium phosphate fast-setting cements and in particular a process for improving the drying and milling of a mixture of a porous material and an aqueous P2 O5 material. It is a further object of this invention to ensure that the dried P2 O5 material absorbed on porous material is a preferred mesh size.
Other objects and advantages of the present invention are described elsewhere within this specification.
In accordance with the present invention a process for preparing a particulate solid activator comprising a porous material containing absorbed P2 O5 values has now been discovered, said process comprises spray drying a slurry of a liquid P2 O5 material and the porous material to obtain said particulate solid. In this process, a preferred porous material comprises diatomaceous earth, and a preferred P2 O5 material comprises an ammonium phosphate solution or alternatively a mixture of phosphoric acid and an ammonium phosphate solution.
Novel, dry, fast-setting magnesium phosphate cementitious compositions have previously been disclosed in U.S. patent application Ser. No. 617,317, filed June 7, 1984. These compositions generally comprise two essential constituents: a solid activator and a magnesium-containing component. The solid activator constituent is a composite comprising a P2 O5 material absorbed onto a porous material. This activator can be prepared by mixing a porous material with a liquid phosphorus pentoxide material then heating the mixture until a dry solid is produced. The dry solid is then milled. An aqueous solution of sequestered ammonium phosphate fertilizer is the preferred P2 O5 material. This invention discloses an improved process for preparing a particulate solid activator containing an aqueous P2 O5 material and particularly the preferred sequestered ammonium phosphate fertilizer.
The term "P2 O5 material" is used throughout this specification to describe material containing phosphorus values. The phosphorus content of these materials is usually analyzed and expressed as P2 O5, hence the term "P2 O5 material".
The liquid, phosphorus pentoxide material can be any material containing free or available P2 O5 such as green acid. This material can be selected from various industrial and agricultural chemicals and wastes. Some examples of suitable P2 O5 materials include aluminum phosphate solution; ammonium phosphate solution; calcium phosphate solution; bright dip phosphoric acid from metal polishing processes; phosphoric acid; waste phosphoric acid from agricultural chemical processes; steel phosphatizing sludge acid resulting from the pickling of steel to inhibit corrosion; arsenic sulfide sludge acid resulting from the treatment of P2 O5 waste streams with H2 S to remove arsenic; and various combinations of the above liquids.
The liquid P2 O5 material can further include various acidic phosphorus compounds, e.g., orthophosphoric acid, pyrophosphoric acids, other polyphosphoric acids and salts thereof.
A preferred liquid P2 O5 material is ammonium phosphate fertilizer solution. This fertilizer solution contains a mixture of orthophosphates and condensed phosphate species. In its original aqueous state, this fertilizer undergoes continuous hydrolysis while standing whereby the weight ratios of the different phosphate forms keep changing, usually from high polyphosphate and low orthophosphate contents to low polyphosphate and high orthophosphate contents as the fertilizer ages. The polyphosphates of this fertilizer solution can include pyro-, tri-, tetra-, etc. phosphate species. Because of this continuous hydrolysis, magnesium phosphate cements using liquid ammonium phosphate fertilizer by itself as the P2 O5 component exhibit unpredictable qualities. Usually, when a fresh fertilizer solution is used by itself, the cement mixture does not set at all or only forms a weak cement, and when an older fertilizer solution is used, the cement sets very rapidly with little or no working time to apply the cement. Liquid fertilizer solution upon standing also precipitates hard to handle insolubles, and in cold weather the liquid freezes and gels creating more handling problems.
A particularly preferred fertilizer solution is Poly-N® 10-34-0 ammonium phosphate solution supplied by Arcadian Corporation and used primarily as an agricultural fertilizer. A typical analysis of the Poly-N® brand fertilizer shows 10.0 weight percent ammoniacal nitrogen and 34 weight percent total P2 O5. Usually, 50 to 60 weight percent of the total P2 O5 is polyphosphate. This commercial product contains approximately 2 weight percent of impurities, which do not adversely affect its use in preparing the solid particulate activator. The major impurities comprise approximately 0.5 weight percent iron, 0.5 weight percent sulfur, 0.2 weight percent magnesium (as MgO) and 0.15 weight percent of fluorine. This commercial product also contains approximately 50 weight percent water. Another preferred fertilizer solution is Poly-N® 11-37-0 ammonium phosphate solution supplied by Arcadian Corporation.
Ammonium phosphate solutions containing from about 5 to about 50 and preferably from about 30 to about 40 weight percent of phosphates expressed as P2 O5 can be used.
When the fertilizer solution is absorbed onto a porous material and heated to dryness to form the activator, the fertilizer is held in a bound form and this composition is much more thermally stable in a temperature range of from ambient to about 120°C, then the original fertilizer solution. The resulting dry solid activator maintains a constant phosphate distribution between the ortho- and polyphosphate species and does not hydrolyze as the original fertilizer solution would.
The drying of the mixture of porous material and aqueous solution of ammonium phosphate to prepare the activator is a complicated step which involves the loss of water, the loss of ammonia, a drop in pH of the mixture and hydrolysis of the pyrophosphate species. In this drying step, water is normally driven off first, however it has been found that the resulting material can have a sticky characteristic which creates fouling problems during milling. This fouling problem can be so severe as to render the material incapable of being milled. The pH of the mixture before drying is usually from about 5.0 to 7.0 and after drying is usually from about 4.0 to about 6∅ One attempt at solving this problem was to heat the sticky material further to reduce its stickiness. During this additional heating step, ammonia is driven off and the pH of the material usually falls to a range of from about 5.0 to about 5.5. This material that has been further heated is now capable of being milled. However, this additional drying step is expensive and the stickiness problem does not occur consistently, therefore process adjustments have to be made in an unscheduled manner.
The mixture of porous material and ammonium phosphate solution that is further heated has been found to have a lower pyrophosphate content. At elevated temperatures, the pyrophosphate is hydrolyzed to orthophosphate.
The improved process of this invention solves the problems outlined above in the preparation of the preferred particulate solid activator. Spray drying can combine three steps in this process: mixing of the liquid P2 O5 material and the porous support material; drying of the resulting slurry; and milling of the resulting dried solid to obtain a preferred particulate solid activator.
In this invention, an acid, preferably phosphoric acid, can be mixed with the preferred ammonium phosphate solution to improve the spray drying process. A preferred mixture comprises from about 20 to about 50 weight percent of phosphoric acid and from about 50 to about 80 weight percent of the ammonium phosphate solution.
The porous material which can be employed in preparing the particulate activator of this invention includes diatomaceous earth, kieselguhr, artificially prepared porous silica, vermiculite, alumina and such naturally occurring substances as various fuller's earth and clays such as bentonite, montmorillonite, acid treated clays and the like. Each porous material which may be used will exert its own specific influence upon the solid particulate activator formed and the fast-setting cement resulting from the use of this activator will not necessarily be identical with that of other members of the class. The porous material can be inert or partially inert. Members of the class of porous material can be used separately or in combination with each other.
A preferred porous material is diatomaceous earth. The highly porous diatomaceous earth can absorb much more than 80 percent by weight, based on the weight of diatomaceous earth, of a phosphate-containing liquid, e.g. liquid ammonium phosphate solution, and absorption of 225 percent and above are preferred. Upon spray drying the diatomaceous earth with the preferred liquid ammonium phosphate solution, the solid particulate activator forms.
The diatomaceous earth can comprise from about 30 percent to about 90 percent by weight of the solid activator with about 30 percent to about 65 percent being preferred; can have a density of from 7.0-19.5 pounds per cubic foot (0.112-0.312 grams per cc); can have a water absorption value of from 170 percent to 255 percent by weight; and can have a surface area of from 1-100 M2 /g. It is believed that when diatomaceous earth is mixed with the aqueous solution of ammonium phosphate fertilizer and heated during the spray drying to form the dry solid particulate activator, more than a physical entrapment of the P2 O5 material can occur.
The liquid ammonium phosphate solution is mixed with the porous material at ambient temperature to form a semi-solid, slurry or paste. Before this slurry is spray dried, additional water can be added to adjust the consistency of the slurry.
The ammonium phosphate solution, and the porous material are mixed in the proportions necessary to form a dry solid particulate activator comprising from about 10 percent to about 70 percent by weight of the P2 O5 material and from about 30 percent to about 90 percent by weight of the porous material in the resulting solid particulate activator.
In a preferred process, phosphoric acid is added to a mixture of ammonium phosphate solution and diatomaceous earth. The pH of the resulting slurry can be adjusted to a range of from about 4.0 to about 5.0 using ammonium hydroxide.
The activator of this invention prepared by spray drying collects primarily as a dry powder on the walls of the drying chamber or as a very fine powder in the collection vessel, depending upon the particular materials used in the process. An ideal activator is one which spray dries easily, passing through the cyclone into the receiver; is a uniform flowable powder; provides a strong cement of about 4000 psi compressive strength after 2 hours with reasonable initial setting time of about 10 minutes with a maximum exotherm of about 75°C; and is formulated with relatively inexpensive ingredients.
Initial activator formulations that were spray dried comprising Poly-N® brand fertilizer, superphosphoric acid, and diatomaceous earth gave very strong cements. The activator solids collected on the drying chamber walls were very easily dislodged by light brushing, and a spray dryer equipped with a mechanical rig should have no problem in dislodging the powder. Although these formulations were acceptable, activators prepared by a process where wall deposits do not occur or are minimized would be more advantageous. Varying the process conditions including slurry concentration and temperature, inlet temperature, feed rate and atomizer disk speed did not change a tendency for wall deposits to occur. However, surprisingly and unexpectedly, it was found that when a portion of the Poly-N® fertilizer was substituted for by orthophosphates, e.g. phosphoric acid, the spray drying characteristics were improved.
When spray drying is compared to other drying methods, for example activators prepared by an oven dried process, the spray dried activator appeared to possess superior properties. Oven drying promotes significant loss of ammonia by thermal decomposition of ammonium phosphates and substantial hydrolysis of pyrophosphate to orthophosphate. Spray drying is a milder process and does not cause nearly as much ammonia loss or pyrophosphate hydrolysis. As a result, the initial composition of the activator slurry more closely matches the final spray dried activator than does the initial composition of the activator slurry match the final oven dried activator. It is desirable to have a process where the final composition of the activator is attained simply by adding those components together, without relying on chemical changes during drying which may or may not occur to the extent desired. This can be achievable with spray drying.
Various types of spray drying equipment may be used in the process of this invention. A Buchi Mini Spray Benchtop Spray Dryer and a Niro Mobile Miner Spray Dryer have been used. Typical spray drying conditions using the Niro Mobile Minor Spray Dryer are listed below.
| TABLE |
| ______________________________________ |
| TYPICAL SPRAY DRYING CONDITIONS |
| ______________________________________ |
| Slurry Concentration: ∼29 Wt. % Solids |
| Inlet Temperature: 150-160°C |
| Outlet Temperature: 85-95°C |
| Air pressure to Centrifugal Disk: |
| Full Open |
| Fan Baffle: Full Open |
| Slurry Feed Rate Setting: |
| 3 |
| Yield Per Trial: 130-160 grams |
| ______________________________________ |
The solid particulate activator prepared by the improved process of this invention can then be dry blended with a solid component comprising a magnesium-containing compound to yield a dry blend capable of forming a fast-setting cement at ambient temperatures when reacted with an aqueous component. The solid activator can optionally be further milled and classified prior to being dry blended. This activator material can usually have a particle size range of from 100-325 mesh and a surface area of from 0.1-10 M2 /g.
Generally, the fast-setting cement containing this activator can be used for producing fast-setting concrete structures, e.g., structures suitable for roadway construction and roadway patches or other load bearing uses. However in its broader aspects, this cement is generally applicable to preparing concrete structures with various characteristics, e.g., high or low strength, for a variety of other purposes as well. The characteristic of any particular concrete structure formed can depend upon the weight ratio of the various components, the nature of any aggregate employed, the temperature conditions of application and the curing conditions as well as other factors.
Fast-setting cement can be applied indoors or outdoors to concrete drives, storage yards, warehouse and factory floors to repair and restore damaged surfaces. Fast-setting cement can be used to fill structural cracks in slabs, and repair highway median barrier walls. This type of cement can be used in almost any situation requiring a quick, permanent repair of concrete. In addition, the fast-setting cement can be used for binding and coating metals and woods for structural and flame resistant applications.
The following experiments describe embodiments of the invention. Other embodiments will be apparent to one of ordinary skill in the art from a consideration of this specification or practice of the invention disclosed therein. It is intended that the specification and experiments be considered as exemplary only, with the true scope and spirit of the invention being indicated by the claims which follow the experiments.
The activator made in this experiment is a dry particulate solid comprising a porous material containing absorbed P2 O5 values.
Sixty grams of phosphoric acid (70 weight percent P2 O5), 60 grams of ammonium phosphate fertilizer solution (type 10-34-0 Poly-N® brand fertilizer supplied by Arcadian Corp.), 40 grams of diatomaceous earth, and 500 milliliters of water were placed in a blender and blended to form a slurry. The resulting slurry was spray dried using a Buchi® Mini Spray Dryer equipped with a cooled 0.5 millimeter pneumatic nozzle and operating at an inlet temperature of 117°C, an outlet temperature of 68°C, and a feed rate of 5 milliliters of slurry per minute. 116 grams of a light brown, fluffy solid activator was collected.
Fifty grams of the resulting activator, 3.3 grams of magnesium oxide (active), 23.8 grams of magnesium oxide (dead burned), 25 grams of fine sand, 100 grams of coarse sand, and 28 milliliters of water were mixed at ambient temperature to form a fast-setting magnesium phosphate cement slurry. This slurry was poured into a 2"×2" (5.08 centimeter×5.08 centimeter) cylinder. The slurry hardened to set initially within 3 minutes of mixing, reaching a maximum exotherm of 62°C The compressive strength of the resulting cement was 1800 psi after 2 hours.
Thirty grams of phosphoric acid (70 weight percent P2 O5), 120 grams of ammonium phosphate fertilizer solution (type 10-34-0, Poly-N® brand fertilizer supplied by Arcadian Corporation), 40 grams of diatomaceous earth, and 500 milliliters of water were placed in a blender and blended to form a slurry. The resulting slurry was spray dried under conditions similar to those of Experiment 1. Spray drying was discontinued after 93 grams of the solid activator was was collected.
Seventy-five grams of the resulting activator, 7.5 grams of magnesium oxide (active), 33 grams of magnesium oxide (dead burned), 37.5 grams of fine sand, 150 grams of coarse sand, and 32 milliliters of water were mixed at ambient temperature to form a fast-setting magnesium phosphate cement slurry. This slurry was poured into a 2" (5.08 centimeters) wide by 2 13/16" (7.14 centimeters) long cylinder. The slurry hardened to an initial set within 5 minutes of mixing and reached a maximum temperature exotherm of 63°C The compressive strength of the resulting cement was 4100 psi after 2 hours.
A series of activators with varying formulations was prepared according to the process described in the above experiments. The activators resulting were formulated into fast-settting magnesium phosphate cements. In one experiment the activator was oven dried and in another the activator was heated prior to spray drying. The results are summarized in Table I below:
| TABLE I |
| __________________________________________________________________________ |
| Compression |
| Experiment Method of |
| Drying Activator |
| Strength |
| No. Formulation |
| Drying Characteristics |
| Wt % P2 O5 |
| After 2 Hrs. |
| __________________________________________________________________________ |
| 1 900 g |
| Poly-N ® fertilizer1 |
| Tray Oven |
| Millable 35 4000 |
| 400 g |
| Diatomaceous earth |
| 2 900 g |
| Poly-N ® fertilizer1 |
| Spray Dryer3 |
| Lightly sticks to |
| 35 500 |
| 400 g |
| Diatomaceous earth |
| walls, easily brushed |
| off as a powder |
| 3 900 g |
| Poly-N ® fertilizer1 |
| Heat @ 80°/2d, |
| No wall sticking, |
| 35 4000 |
| 400 g |
| Diatomaceous earth |
| Spray Dryer3 |
| a very fine powder |
| 4 267 g |
| Poly-N ® fertilizer1 |
| Spray Dryer3 |
| No wall sticking, |
| 35 3400 |
| 326 g |
| H3 PO42 |
| a very fine powder |
| 400 g |
| Diatomaceous earth |
| 5 61.4 g |
| Poly-N ® fertilizer1 |
| Spray Dryer3 |
| No wall sticking, |
| 32 3400 |
| 92.0 g |
| Diatomaceous earth |
| a very fine powder |
| 75.0 g |
| H3 PO42 |
| __________________________________________________________________________ |
| 1 10-34-0 supplied by Arcadian Corporation. |
| 2 The H3 PO4 contained 85 weight percent P2 O5 |
| and the slurry was adjusted to a pH of 4.5 with ammonium hydroxide. |
| 3 Niro Mobile Minor Spray Dryer. |
The data in the above Table demonstrate that solid particulate activators can be made according to the process of this invention. These activators can be formulated into effective magnesium phosphate fast-setting cements.
Band, Elliot I., Sherif, Fawzy G.
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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
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| Oct 11 1985 | SHERIF, FAWZY G | Stauffer Chemical Company | ASSIGNMENT OF ASSIGNORS INTEREST | 004853 | /0925 | |
| Oct 15 1985 | Stauffer Chemical Company | (assignment on the face of the patent) | / |
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